1. Field of the Invention
The present invention relates to a method for grinding a journal section of a workpiece with a steady rest device being in contact at its rest shoe with a part of the workpiece to decrease the flection of the workpiece.
2. Discussion of the Related Art
In a grinding field of crankshafts for automotive engines, it has been a practice that such a crankshaft is ground at its crankpins while being supported by a work spindle of a grinding machine. In grinding each of the crankpins, a rest shoe of a steady rest device is brought into contact with a cylindrical surface of a journal section which is at a position shifted in the axial direction from a crankpin to be ground, as described in U.S. Pat. No. 6,409,573 (equivalent of Japanese unexamined, published patent application No. 2000-296444) for example. The steady rest device decreases the flection of the crankshaft which flection is caused by grinding resistance applied from a grinding wheel, so that the machining accuracy of the crankpin can be enhanced.
A journal section of the crankshaft is ground in the manner illustrated in FIG. 5. That is, in grinding a journal section J1 on one end side of the crankshaft W with a grinding wheel 17, the grinding of the journal section J1 is performed with a rest shoe 34 of a steady rest device 30 being in contact with a center journal section J3 of the crankshaft W. It is often the case that the grinding of the journal section J1 is performed with a sizing device measuring the diameter of the journal section J1 and the width between shoulder surfaces at the opposite ends of the journal section J1. Where the grinding of the journal section J1 is performed using the sizing device, it becomes difficult to bring the rest shoe 34 into contact with the journal section J1 being ground, and thus, the grinding is practically performed with the rest shoe 34 being in contact with another journal section J3 which is axially spaced from the journal section J1 being ground.
First of all, description will be made regarding a cylindrical grinding machine which is employed for an exemplified grinding method (hereafter referred to as “grinding method in a compared example”) for grinding the journal section J1 with the rest shoe 34 of the steady rest device 30 being in contact with the cylindrical surface of the center journal section J3 of the crankshaft W. As shown in FIG. 5, at the rear part of an upper surface of a bed 10 of the cylindrical grinding machine, a pair of guide rails 11 are secured in a horizontal left-right direction (Z-direction), and a feed table 12 is supported and guided on the guide rails 11. Another pair of guide rails 13 are secured on the feed table 12 in a horizontal front-rear direction (X-direction), and a wheel head 14 is supported and guided on the guide rails 13. A wheel spindle (not shown) extending in the Z-direction is rotatably carried in the wheel head 14 and is drivingly rotated by a grinding wheel motor (not shown), built in the wheel head 14, together with a grinding wheel 17 secured to one thereof. As shown in an enlarged fragmentary view of a grinding area encircled in FIG. 5, the grinding wheel 17 is of the configuration that an annular grinding wheel layer in which CBN abrasive grains have been bonded with a vitrified bonding agent is securely provided on the circumferential surface of a grinding wheel core made of, e.g., a metal disc and that the thickness in the axial direction of the grinding wheel layer is somewhat greater than that of the grinding wheel core.
A Z-axis servomotor 15 attached to the bed 10 is drivingly controllable by a numerical controller 18 and feeds the feed table 12 and the wheel head 14 and the grinding wheel 17, which are supported on the feed table 12, through a feed screw 15a in the Z-direction. An X-axis servomotor 16 attached to the feed table 12 is drivingly controllable by the numerical controller 18 and feeds the wheel head 14 and the grinding wheel 17, supported on the wheel head 14, through a feed screw 16a in the X-direction. The respective servomotors 15, 16 are provided with encoders, which respectively detect the positions of the feed table 12 and the wheel head 14 to feed the detected positions back to the numerical controller 18.
A work table 20 is fixed at the front part on the operator side (i.e., lower side in FIG. 5) of the upper surface of the bed 10 of the grinding machine, and a work head 21 rotatably carrying a work spindle 22 and a foot stock 23 are provided on the work table 20 in axial alignment to face each other in the Z-direction. Centers 22a, 23a provided on the work spindle 22 and the foot stock 23 support the opposite ends of the crankshaft (i.e., workpiece) W. The work spindle 22 is drivingly rotatable by a work spindle servomotor 24, which is mounted on the work head 21 to be controllable by the numerical controller 18. The crankshaft W is rotated together with the work spindle 22 with its left end portion engaged with a driving dog (not shown) secured to the work spindle 22. The work spindle servomotor 24 is also provided with an encoder, which detects the rotational position of the work spindle 22 to feed the detected position back to the numerical controller 18.
The crankshaft W is a one-body article, which has five journal sections J1-J5 arranged in axial alignment with a space between each journal section and the next thereto, four pairs of crank arms CA radially extending at opposite end portions of the respective journal sections J1-J5 in parallel relation and four crankpins P1-P4 each jointing the radial outmost end portions of an associated pair of crank arms CA. A large-diameter portion K is formed at one end portion of the crankshaft W which portion is outside a first journal section J1 at the leftmost as viewed in FIG. 5, so that the first journal section J1 has a cylindrical surface S1 and a pair of shoulder surfaces S2, S3 which extend from the opposite end portions of the cylindrical surface S1 radially outward.
A base 31 of the steady rest device 30 is fixed on an operator side edge portion of the bed 10 which portion is on a side opposite to the wheel head 14 with the crankshaft W therebetween. A rest head 32 is supported and guided on the base 31 to be movable in the Z-direction. A servomotor 33 attached to the base 31 is drivingly controllable by the numerical controller 18 and feeds the rest head 32 through a feed screw 33a in the Z-direction. A rest shoe 34 which is supported and guided by the rest head 32 to be movable in the X-direction is moved by a servomotor 35 back and forth between predetermined advanced and retracted positions. The respective servomotors 33, 35 are provided with encoders, which respectively detect the positions of the rest head 32 and the rest shoe 34 to feed the detected positions back to the numerical controller 18.
Next, with reference to FIGS. 5 to 7, description will be made regarding the grinding method in the compared example which is implemented in the grinding machine as constructed above. In this compared example, the feed table 12 is moved by the Z-axis servomotor 15 and positions the grinding wheel 17 drivingly rotated by the built-in grinding wheel motor (not shown), to a position where the grinding wheel 17 comes to align with and face the first journal section J1 of the crankshaft W which has been center-supported by the work spindle 22 and the foot stock 23. Then, the wheel head 14 is moved by the X-axis servomotor 16 to make the grinding wheel 17 approach the crankshaft W, whereby a shoulder grinding is first performed with opposite end surfaces of the grinding wheel 17 to simultaneously grind the left and right shoulder surfaces S2, S3 of the first journal section J1 and whereby a cylindrical grinding is then performed with the circumferential surface of the grinding wheel 17 to grind the cylindrical surface S1 of the first journal section J1. Before the feed of the wheel head 12 toward the crankshaft W, the servomotor 33 of the steady rest device 30 is operated by the numerical controller 18 to position the rest head 32 to a position where the rest shoe 34 is aligned with a third journal section J3 (another journal section) which is at a position different axially from the first journal section J1 on the crankshaft W. As is a practice in the grinding field, for a small quantity of allowance in the grinding operation, a pre-machining groove Sa (shown in FIG. 7(a)) has been formed at the position of the first journal section J1 of the blank of the crankshaft W through a preceding step such as, e.g., turning, milling or the like. The pre-machining groove Sa has an axial width which is somewhat narrower than the axial width of the annular grinding wheel layer of the grinding wheel 17.
In the inoperative state, as shown in FIG. 5, the grinding wheel 17 is away from the first journal section J1 of the crankshaft W, and the rest shoe 34 of the steady rest device 30 is at a retracted position where it is away from the circumferential surface of the third journal section J3. Thus, the axial center CL of the crankshaft W extends in parallel to the Z-axis, as shown in FIG. 7(a). In this state, the work spindle servomotor 24 is operated by the numerical controller 18 to rotate the work spindle 22 and the crankshaft W supported thereby, and as indicated by the solid line A in FIG. 6, the wheel head 14 is advanced at a rapid feed rate, whereby the grinding wheel 17 advancing together with the wheel head 14 approaches the first journal section J1 of the crankshaft W.
Somewhat before the circumferential surface of the grinding wheel 17 reaches the shoulder surfaces S2, S3 of the first journal section J1, the numerical controller 18 operates the servomotor 35 of the steady rest device 30 to advance the rest shoe 34 to an advanced position where the rest shoe 34 comes into contact with the external surface of the third journal section J3 of the crankshaft W. Thus, the axial center area of the crankshaft W is somewhat flexed toward the wheel head 14 side, and this causes the first journal section J1 to tilt counterclockwise, as shown in FIG. 7(b). Although FIG. 7 depicts the tilt in an exaggerated scale for ease to see, the flection that the push by the rest shoe 34 brings about in the neighborhood of the third journal section J3 of the crankshaft W is in a range of several ten-micron meters, and the displacement which the flection gives to the maximum diameter portion of each shoulder surface S2, S3 extending perpendicular to the axial center CL at the first journal section J1 of the crankshaft W is as extremely small as several micron meters or so.
With the advance of the wheel head 14, the circumferential surface of the grinding wheel 17 advancing with the wheel head 14 reaches the shoulder surfaces S2, S3 of the first journal section J1, and the numerical controller 18 then operates the X-axis servomotor 16 to switch the feed rate of the wheel head 14 from the rapid feed rate to a shoulder grinding feed rate slower than the rapid feed rate, whereby the grinding of the left and right shoulder surfaces S2, S3 begins. During the shoulder grinding, small grinding resistance is generated, and this causes the first journal section J1 to tilt slightly clockwise from the state shown in FIG. 7(b). In this state, the shoulder grinding indicated by the solid line B in FIG. 6 is performed to grind the left and right shoulder surfaces S2, S3, as shown in FIG. 7(c).
The further advance of the wheel head 14 makes the shoulder grinding progress. When the circumferential surface of the grinding wheel 17 reaches a bottom surface of the pre-machining groove Sa of the first journal section J1 as shown in FIG. 7(c), the feed rate of the wheel head 14 is switched to a cylindrical grinding feed rate slower than the shoulder grinding feed rate in the same manner as described above, whereby the grinding indicated by the solid line C in FIG. 6 is initiated on the cylindrical surface S1 of the first journal section J1. During this cylindrical grinding, the grinding resistance increases to be considerably greater than that during the shoulder grinding. The first journal section J1 is tilted clockwise from the state shown in FIG. 7(c), and the cylindrical grinding indicated by the solid line C in FIG. 6 is performed in this state. In this cylindrical grinding, due to the clockwise tilt, the shoulder surface S3 on the left side is ground with the left end surface of the grinding wheel 17 to a larger depth as shown in FIG. 7(d) than it was ground in the state of FIG. 7(c), and a clearance is made between the shoulder surface S2 on the right side and the right end surface of the grinding wheel 17. The cylindrical grinding is subdivided into a first rough grinding (solid line C1), a second rough grinding (solid line C2), a fine grinding (solid line C3) and a minute grinding (solid line C4) wherein the feed rate of the wheel head 14 are in turn reduced stepwise and also into a spark-out grinding (indicated by the first dotted circle at the end of the solid line C4 in FIG. 6) which is performed, with the infeed of the wheel head 14 being stopped, in succession to the minute grinding. The whole operation in the cylindrical grinding is as described above.
To follow the spark-out grinding on the cylindrical surface S1, the wheel head 14 is retracted by a predetermined or fixed distance, as indicated by the solid line D in FIG. 6. If the retraction of the grinding wheel 17 at a rapid feed rate were performed immediately after the grinding of the cylindrical surface S1 is completed, the flection of the crankshaft W would be released at a moment, resulting in a further infeed of a part of the cylindrical surface S1 against the grinding wheel 17, whereby the roundness of the finished cylindrical surface S1 would be degraded in accuracy. The retraction of the fixed distance indicated by the solid line D in FIG. 6 is to prevent the cylindrical surface S1 from being degraded in roundness by retracting the grinding wheel 17 by the predetermined or fixed distance at a slow feed rate, and the distance or amount of the retraction is a small amount. Thus, even though the retraction by the fixed distance decreases the grinding resistance, there remains grinding resistance of the same degree as that in the shoulder grinding. The remaining grinding resistance thus causes the first journal section J1 to tilt counterclockwise from the state shown in FIG. 7(d). As a consequence, as shown in FIG. 7(e), the clearance which has been made between the shoulder surface S2 on the right side and the right end surface of the grinding wheel 17 during the cylindrical grinding is reduced to zero (0), whereas a clearance is made between the shoulder surface S3 on the left side and the left end surface of the grinding wheel 17.
After being stopped momentarily (as indicated by the second dotted circle in FIG. 6) at the retracted end of the fixed-distance retraction indicated by the solid line D in FIG. 6, the wheel head 14 is further retracted at a semi-rapid feed rate as indicated by the solid line E. During this retraction state, the grinding resistance caused by the grinding wheel 17 becomes zero (0) to return to the state shown in FIG. 7(b). Thus, the tilt of the first journal section J1 toward the counterclockwise direction which tilt is brought about by the rest shoe 34 remaining in contact with the third journal section J3 becomes larger in value than that during the shoulder grinding shown in FIG. 7(c). As a result, during the retraction at the semi-rapid feed rate, the grinding wheel 17 is retracted with the right end surface thereof interfering with the shoulder surface S2 on the right side of the first journal section J1. This causes vortex or spiral-like shallow scrapes to be formed at the shoulder surface S2 of the first journal section J1. This could make grinding burn on the shoulder surface S2 in the case of the grinding wheel 17 being low in sharpness. Also during the retraction, a clearance is made between the left end surface of the grinding wheel 17 and the shoulder surface S3 on the left side of the first journal section J1.
When the semi-rapid feed retraction of the wheel head 14 makes the circumferential surface of the grinding wheel 17 go away radially outside from an area facing the shoulder surfaces S2, S3 of the first journal section J1, the numerical controller 18 operates the servomotor 35 of the steady rest device 30 whereby the rest shoe 34 is retracted toward the retracted position where it is away from the external surface of the third journal section J3. Thus, the axial center CL of the first journal section J1 of the crankshaft W is brought into a parallel relation with the Z-direction, as shown in FIG. 7(g). In this state, the wheel head 14 is retracted at the rapid feed rate indicated by the solid line F in FIG. 6 to return to the initial inoperative state mentioned in the beginning of this operational description.
As described above, in the compared example shown in FIGS. 5-7, there arises a problem that spiral shallow scratches or scrapes or, on a certain occasion, grinding burn is formed on the shoulder surface S2 of the first journal section J1 when the grinding wheel 17 is retracted at the semi-rapid feed rate following the fixed-distance retraction.